This instrument is used to measure the kinetics of CO binding to cytochromes P450 in liver microsomes from rats treated with various drugs and carcinogens. When CO is added directly to rat liver microsomes, the absorbance change at 450 nm occurs too rapidly to follow on a standard spectrophotometer. In order to observe this rapid reaction, a continuous dye-laser flash photolysis apparatus was constructed to monitor the kinetics of the absorbance change.